An emerging theme in the pathogenesis of human malignancy is the involvement of tyrosine kinases as a consequence of chromosomal translocation. However, the mechanism of transformation of this class of fusion proteins is not well understood, even for the extensively studied BCR-ABL fusion associated with t (9;22) chronic myelogenous leukemia. Our laboratory has recently cloned two new tyrosine kinase fusions, TEL- PDGFRbeta and TEL-ABL, which are associated with t (5;12) and t(9;12) chromosomal translocations respectively, in human leukemias. TEL is a new member of the ETS family of transcription factors, and contributes a highly conserved helix-loop-helix (HLH) domain to both fusions. The objective of this proposal is to determine the mechanism of transformation of TEL- PDGFRbeta and TEL-ABL by analysis of the contribution of the functional domains of TEL, PDGFRbeta and ABL. We hypothesize that transformation is due to aberrant expression and activation of the PDGFRbeta and ABL tyrosine kinases mediated by the TEL HLH domain. Preliminary studies have documented the transforming activity of the fusion proteins, and demonstrate that tyrosine kinase activity of PDGFR and ABL are absolute requirements for transforming activity. The function of the HLH domain in ETS family members is unknown. However, preliminary analysis of these fusions has led us to an interesting discovery: the TEL HLH domain appears to be a protein-protein interaction motif. This finding, in conjunction with the observation that the other TEL allele is deleted in TEL-ABL leukemia and expressed at low levels in TEL-PDGFRbeta leukemia, suggests that TEL loss of function may be important in pathogenesis of leukemia. In Specific Aim 1, we will characterize the signal transduction properties of the TEL-PDGFRbeta fusion protein, and determine which are pivotal in mediating transformation. In Specific Aim 2, we will analyze the signal transduction properties of TEL-ABL, and determine the common themes which unify the transforming properties of TEL-ABL and BCR-ABL. Specific Aim 3 will focus on the role of the TEL HLH domain in transformation and the possibility that loss of function of TEL contributes to pathogenesis of leukemia. Delineation of the mechanism of transformation of TEL-PDGFRbeta TEL-ABL will be facilitated by interactions with each of the members of the Program Project, and may provide insights into therapeutic approaches to human malignancy caused by tyrosine kinase fusions.